Achieving a stable enhancement-mode (E-mode) operation with a sufficiently high threshold voltage (Vth) remains a key challenge for GaN power HEMTs. This work investigates the effects of ferroelectric stack thickness and annealing sequence on GaN MOS-HEMTs grown on SiC substrate incorporating a hybrid ferroelectric gate stack composed of Al2O3/HZO/Al2O3 with an embedded HfON charge-trap layer between HZO and Al2O3 layers. Devices with different HZO stack thicknesses were subjected to postdeposition annealing (PDA) or postmetallization annealing (PMA) at 400 °C in N2 ambient. PMA-treated devices exhibit a higher and more stable Vth, significantly reduced C–V dispersion, and improved robustness after gate-bias initialization compared with PDA-treated devices. The optimized PMA device with a thicker ferroelectric stack achieves a positive Vth of up to 5.39 V, enabling a programmable E-mode operation. In addition, ferroelectric metal-ferroelectric-metal capacitors demonstrate pronounced hysteresis behavior in PMA-treated samples, with a remanent polarization (2Pr) of 36.63 μC/cm2. These results demonstrate that annealing-sequence-controlled hybrid ferroelectric gate engineering is an effective strategy for realizing high-Vth, stable enhancement-mode GaN HEMTs.
Liao et al. (Fri,) studied this question.